Sometimes generating an ideas is a matter of seeing what others have done.  The following examples were found by searching Google and DuckDuckGo for "arcgis storymap flood risk".

The paper "Flood Risk Communication Using ArcGIS StoryMaps" details the development of Calgary's River Flood Story.

Monica detailed her experience using ArcGIS StoryMaps in a report titled "The Everyday Risk of Flooding:  An Interactive Story Map Tackling Common Flooding Misconceptions."

Bradley Dean created a fun slide deck "Communicating Flood Risk Using Esri Story Maps."

The following examples were found by searching Google for "arcgis storymap watershed".